Here we present first principles calculations of new functional
phases of BiFeO$_3$ and BiMnO$_3$ stabilized by strain in thin
film geometries. In BiFeO$_3$ we examine a supertetragonal-like
phase recently identified in films grown on LaAlO$_3$ and
YAlO$_3$ substrates.\footnote{B\'{e}a {\em et al.}, Phys. Rev.
Lett. {\bf 102}, 2009.}$^,$\footnote{Zeches {\em et al.}, Science
{\bf 326}, 2009.} We characterize the transition as
a function of strain and find that it occurs between phases that
are isosymmetric yet have dramatically different structures and
properties, the most notable of which is a strong enhancement and
rotation of the electric polarization.\footnote{Hatt {\em et
al.}, arXiv:0909.4979.} From our
computational results we suggest a model for the development of
coexisting phases found in BiFeO$_3$ films on LaAlO$_3$
substrates.\footnote{Zeches {\em et al.}, Science
{\bf 326}, 2009.} In BiMnO$_3$ we demonstrate strain stabilization
of an orbital ordering that gives rise to unusual ferroelectric
and magnetic properties. [Preview Abstract]

Previously, we had reported on the universal trend in structural
transition in rare-earth-doped BiFeO$_{3}$ films grown on (100)
SrTiO$_{3}$ substrates [1]. At this boundary, we observe double
hysteresis loops in PE hysteresis loops due
to an electric-field-induced transition and substantial
enhancement in dielectric constant and piezoelectric coefficient.
In this study, we have
investigated substrate dependence of the structural and
ferroelectric properties of (Bi,Sm)FeO$_{3}$ by growing the
composition spreads on various substrates. We chose (100), (110)
and (111) SrTiO$_{3}$, (100) LSAT and (110) DyScO$_{3}$ as
substrates. Scanning x-ray diffraction reveals that all the
fabricated spread films exhibit the rhombohedral to orthorhombic
structural transition at Sm 14\%, independent on the type of the
substrates. This confirms that the observed structural transition
is a bulk property of the (Bi,Sm)FeO$_{3}$ system. The detailed
correlations between the structural properties and ferroelectric
properties depending on the type of the substrate will be
discussed. This work is supported by DMR, NSF MRSEC DMR, ARO and
the W. M. Keck
Foundation.\\[4pt]
[1] S. Fujino et al., APL 92, 202904 (2008). [Preview Abstract]

Samarium doped bismuth ferrite (BSFO) is a good lead free candidate for
piezoelectric applications. For Sm concentrations of $\sim $14{\%}, BSFO is
at a morphotropic phase boundary (MPB) between two structural phases much
like lead zirconium titanate (PZT). We have conducted a high resolution
synchrotron x-ray diffraction study of Sm doped BiFeO3 films at a
composition near the MPB. Films of varying thicknesses were studied over a
temperature range of 25C to 700C. We found that the mixed phases associated
with the MPB vary with temperature and thickness, indicating that they
undergo a transition near 275C. Fundamental lattice constants were measured
as a function of temperature; additionally this allowed us to probe
structural twins. Also, thermal history dependence was found that indicates
a chemical change occurring above film growth temperature. [Preview Abstract]

Recent study suggests that in BiFeO$_{3 }$ (BFO) thin films
epitaxial strain can lead to an isostructural change from
distorted rhombohedral to a distorted tetragonal structure. In
this work, ferromagnetic (or pyromagnetic) phase boundaries
between these two phases in BFO thin films are induced from the
as-grown state or by applying electric field using piezo-
response force microscopy. Rhombohedral-like BFO is a G-type
antiferromagnet with its easy plane in (111)-type planes. In
depth density functional theory calculations suggest that
tetragonal-like BFO is a C-type antiferromagnet with its easy
plane in (110)-type planes. Detailed antiferromagnetic ordering
of tetragonal-like BFO has been carefully studied using photo-
emission electron spectroscopy with linear polarized x-rays.
The isostructural phase boundary is magnetically frustrated
because the transition between G-type and C-type
antiferromagnetic ordering. X-ray magnetic circular dichroism
is observed using photo-emission electron microscopy and
spectroscopy with circular polarized x-ray. [Preview Abstract]

Materials with morphotropic phase boundary (MPB) compositions attract
people's interests because of their huge electromechanical response. The
ambiguity of crystal structure, phases and physics behind presents a
challenge to interpret the origin of abnormally high piezoelectric
coefficient. Recently, epitaxial strain can be used to stabilize the MPB in
BiFeO$_{3}$. [1] However, to better understand the origin of MPB in
multiferroic perovskite, it is essential to probe into the
strain/polarization coupling mechanism. In this abstract, origin and
dynamics of the phase transition in BiFeO$_{3}$ near MPB have been
investigated by exploring the ferroelectric domains. Careful analysis of
domain configurations across the phase boundary reveals the rotation of
ferroelectric vectors from phase to phase, a new strain/polarization
coupling state in nature. The phase transition dynamics are assisted by the
re-orientation of ferroelectric polarizations. Phase-field simulation
further proves the origin of this MPB is the strain-driven rotation and
re-orientation of ferroelectric vectors. This investigation provides new
insight into the study of high electromechanical response in MPB-like
crystal and helps engineer other lead free-ferroelectrics.
Reference:
1. R. J. Zeches, et al., Science 326, 977 (2009). [Preview Abstract]

We measured the optical properties of tetragonal thin film and
nanoscale rhombohedral BiFeO$_3$ in the range from near
infrared to the near ultraviolet. The absorption spectrum in
the tetragonal film is overall blue-shifted compared with that
of the rhombohedral BiFeO$_3$ film. It shows an absorption
onset near $2.25$ eV, a direct $3.1$ eV band gap, and charge
transfer excitations that are $\sim$0.4 eV higher than those of
the rhombohedral counterpart. In the nanoparticles, the band
gap decreases from $2.7$ eV to $\sim$2.3 eV, and the well-known
$3.2$ and $4.5$ eV charge transfer excitations split into
multiplets. We discuss these results in terms of structural
strain, surface strain, and local symmetry breaking. [Preview Abstract]

In the first-principles search for new
ferromagnetic-ferroelectric multiferroics,
one key indicator is the softening of the lowest frequency polar
phonon with ferromagnetic ordering from a paraelectric
antiferromagnetic bulk state. In a first-principles survey of the
phonon dispersions of a wide range of magnetic perovskites, we
identified SrMnO$_3$ as a promising candidate system. We find
that a ferromagnetic-ferroelectric phase is stabilized by both
compressive and tensile epitaxial strain. For compressive strain,
there is a sequence of intermediate magnetic transitions, first
to $C$-AFM and then to $A$-AFM ordering, with an increasing
fraction of ferromagnetically aligned nearest neighbor Mn. At
each of these, the change in magnetic order is accompanied by a
jump in the magnitude of the electric polarization, so, near the
$A$-AFM-FE$\rightarrow$FE-FM phase boundary at 3.4\% and
$G$-AFM-FE$\rightarrow$FE-FM phase boundary at -2.9\%, an applied
electric field can induce a nonzero magnetization, and the jump
in $c$-lattice constant at -2.9\% strain can generate a large
piezomagnetic response. The origin of the large phonon softening
in SrMnO$_3$ will be examined, which should provide guidance in
identifying additional candidate systems for
epitaxial-strain-induced multiferroicity. [Preview Abstract]

The critical thickness has been a fundamental issue in thin films of
ferroelectric materials--the absence of which was recently predicted
theoretically for improper ferroelectric materials of hexagonal
YMnO$_{3}$
(YMO). [1] Here we report second harmonic generation (SHG)
studies on YMO
ultrathin films, revealing experimentally this prediction.
The YMO ultrathin films were deposited on YSZ (111) substrates by
pulsed
laser ablation. The samples were single crystalline having
atomically flat
surfaces and epitaxial relation ([10-10]$_{YMO
}$//[-110]$_{YSZ})$, as
characterized by \textit{in-situ} RHEED, AFM, XRD, and HR-TEM. It
is found that even a film
with 1.5-unit-cell thickness shows substantial SHG intensity,
indicating the
existence of spontaneous polarization. In addition to the
polarization along
the $c$-axis, we found in-plane components which are not observed
in single
crystals. The details of the crystallographic symmetry and
possible origins
of the in-plane components will be discussed.
\\[4pt]
[1] N. Sai, \textit{et al}., Phys. Rev. Lett. \textbf{102},
107601 (2009). [Preview Abstract]

With recent breakthroughs in fabricating high-quality oxide
films, ultra
thin ferroelectric (FE) films have attracted significant
attention. Many
FE-based electronic devices proposed to date have a capacitor
configuration,
where a FE layer is inserted between two identical metal
electrodes. We
consider theoretically so-called tricolor structures or asymmetric
capacitors with one electrode being ferromagnetic and other
normal metal. An
interesting aspect of a tricolor structure is breaking of the
inversion
symmetry which is expected to generate new properties. Of particular
interest is the control of the magnetization in a ferromagnetic
layer
without using an external magnetic field. The effect may find
applications
in low-power and high-density integration in future spintronics
devices. To
investigate the polarization-dependent magnetization change in
the iron
layer we construct the tricolor superlattices comprised of
Fe/BaTiO$_{3}$/Pt, Fe/PbTiO$_{3}$/Pt and perform first principles
calculations at the LSDA+U level. We find the electrode
magnetization
sensitive to the polarization direction in the FE layer, which
suggests a
multiferroic character of the structure. The effect is much
stronger than in
the analogous symmetric structures. We also investigate the
change of the
depolarization field and screening length due to the lattice
relaxation. [Preview Abstract]

Rock salt binary AO oxides form an important family of compounds
which was intensively studied, both experimentally and
theoretically. In comparison to multifunctional ferroelectric
perovskite oxides, their practical applications remain however
limited and the emergence of ferroelectricity and related
functional properties in simple binary oxides seems so unlikely
that it was never previously considered.
Here, we first show from first-principles density functional
calculations that ferroelectricity can be easily induced in
simple alkaline earth binary oxides such as BaO using appropriate
epitaxial strains. We point out that the functional properties
(polarization, dielectric constant and piezoelectric response) of
such strained binary oxides are comparable in magnitude to those
of typical ferroelectric perovskite oxides, so making them of
direct interest for applications.
Going further, we also show the possibility to induce
ferroelectricity in the ferromagnetic rock salt binary oxides
EuO, and so suggesting a new route to achieve multiferroism at
high temperature by combining ferroelectric and magnetic
properties in very simple structures. [Preview Abstract]

Anisotropy of electrical and magnetic properties in
magnetoelectric and
multiferroic materials is an important issue for applications of the
materials to electronic devices. Ga$_{2-x}$Fe$_{x}$O$_{3}$ (GFO)
has been
known as a pyroelectric ferrimagnet at room temperature when x
$>$ 1.4. GFO
exhibits a permanent polarization along $b$-axis while a
spontaneous net
magnetization along $c$-axis. Exploration of its anisotropic
properties
requires preferentially oriented epitaxial thin films of GFO with
different
crystalline orientations. We have grown successfully $b$-axis
oriented GFO thin
films on indium-tin oxide(001)/yttria-stabilized zirconia(001). Two
additional bottom electrodes such as SrRuO$_{3}$ on
SrTiO$_{3}$(111), (110)
and (100) and Pt(111)/Ti/SiO$_{2}$/Si substrates were used for
epitaxial
growth of GFO. X-ray diffraction and transmission electron
microscopy have
been performed. Dielectric permittivity of the GFO films was
measured with
external magnetic field as a function of temperature. Local
polarization
switching behavior was characterized by scanning probe
microscopy, which can
give a clue to answer a debating question that GFO thin films are
pyroelectric with no bistable switching states. [Preview Abstract]

EuO is an interesting material for magnetism and spintronics
applications due to its large magneto-optic effects,
metal-to-insulator
transition, and spin splitting of the conduction band. While bulk
EuO was
extensively studied as long as 40 years ago, epitaxial and
stiochiometric
films have been difficult to produce. We report the growth of the
magnetic
insulator EuO on Yttrium Stabilized Zirconia (YSZ 100) and GaAs
(001) by
reactive Molecular Beam Epitaxy (MBE). The growth is achieved in
a regime in
which excess Eu ions are re-evaporated and the ratio of Europium
to Oxygen
flux is more than one. Magneto-Optic Kerr Effect (MOKE)
demonstrates a
transition for all films at 69K. Further, we investigate the
optical and
transport properties of these films as well as temperature
dependence.
[Preview Abstract]